This invention relates in general to systems that include solenoids and in particular to an apparatus and method for detection of small undesired solenoid currents.
A typical prior art control circuit 10 used for multiple solenoids is shown in FIG. 1. The circuit 10 has a common high side Field Effect Transistor (FET) 12 with a drain terminal connected to a power supply V+. The source terminal of the high side FET 12 is connected to a high end of each of a plurality of solenoid coils labeled L1 through LN. The low end of each of the solenoid coils is connected to the drain of an associated low side FET, which is correspondingly labeled T1 through TN. The source of each of the low side FETs is connected to ground. The control circuit 10 also includes a plurality of resistors that are labeled R1 through RN, whose resistance is significantly higher than that of the solenoid coils. Each of the resistors R1 through RN is connected in parallel with a corresponding coil L1 through LN, respectively to permit detection of an open coil. The gates of the high side FET 12 and the low side FETs T1 through TN are connected to a controller 14 through an appropriate circuit (not shown) to adapt the controller output voltage level to the voltage level required to switch the FETs. The controller typically includes a microprocessor and an algorithm. The microprocessor is responsive to the algorithm to generate signals to selectively switch the individual FETs between their conducting and non-conducting states. Switching the high side FET 12 to its conducting state provides power to all of the low side FETs T1 through TN, such that switching the low side of a selected FET to a conducting state enables current to flow through the associated solenoid coil.
The circuit 10 further includes a plurality of voltage feedback circuits which are used to detect undesired activations of a solenoid or failures of a solenoid to activate when desired. One high side voltage feedback circuit is shown to the left of FIG. 1 as a voltage divider circuit VDF that monitors the voltage provided to the high sides of the solenoid coils L1 through LN. The resistance of the voltage divider circuit VDF is significantly higher than that of the solenoid coils. The feedback voltage is labeled VFBY and is provided to the controller 14. When the high side FET 12 is in a conducting state, a high feedback voltage VFBY will appear at the midpoint of the voltage divider VDF. When the high side FET 12 is in a non-conducting conducting state, a low feedback voltage VFBY will appear at the midpoint of the voltage divider VDF. While a voltage divider circuit VDF is shown in FIG. 1, the configuration is meant to be illustrative and any other conventional method for monitoring voltage may be utilized.
Also shown in FIG. 1 are a plurality of coil voltage divider circuits, VD1 through VDN, connected between the low end of each solenoid coil and ground. The coil voltage divider circuits VD1 through VDN monitor individual coil feedback voltages that are labeled VFB1 through VFBN with the individual coil feedback voltages being provided to the controller 14. When the high side FET 12 is in its conducting state and a selected low side FET TN is in its non-conducting state, a high feedback voltage VFBN will appear at the midpoint of the associated voltage divider VDN. When the high side FET 12 and a selected low side FET TN are in their conducting states, a low feedback voltage VFBN will appear at the midpoint of the associated voltage divider VDN. If the high side FET 12 and a selected low side FET TN are in their non-conducting states, a low feedback voltage VFBN will appear at the midpoint of the voltage divider VDN. Again, the use of voltage divider circuits VD1 through VDN for monitoring the coil feedback voltages is meant to be illustrative, and other conventional methods for monitoring the coil feedback voltages also may be utilized.
As described above, when both the high side FET 12 and a selected low side FET, such as T2, are switched to their conducting states, a large current flows through the associated solenoid coil L2 and an associated low feedback voltage VFB2 will be provided to the controller 14. When the low side FET T2 is switched to its non-conducting state, a small current will flow through L2 and VD2, and an associated high feedback voltage VFB2 will be provided to the controller 14. In the absence of a fault, the level of the voltage feedback VFBY and VFB2 would indicate that the FETs 12 and T2 and the coil L2 are operating correctly. A fault due to an open or shorted coil L2 or an open, leaky or shorted low side FET T2 can be detected through the voltage feedback VFB2 when the monitored voltages are not as expected, as shown in following table.
VFBYLOW SIDE FET TYFEEDBACKCOMMANDED STATEVOLTAGEONOFFHIGHOPEN FETGOODMIDSHORTED COILOPEN COIL orLEAKY FETLOWGOODSHORTED FET
The resistors R1 through RN, that are connected in parallel with the coils L1 through LN, and the coil voltage divider circuits, VDD through VDN, allow an open coil to be distinguished from a shorted low side FET. With an open coil, the current flows through the corresponding resistor and the resulting voltage drop causes the voltage feedback to be lower than expected when the FET is off, but not so low as it would be if the low side FET is shorted. Although this allows an open coil to be detected, it may be difficult to distinguish an open coil from a leaky FET in certain ranges of current. If the leakage current is very low, there is little voltage drop across the coil and the resulting feedback voltage is close to the supply voltage and thus may not be detected. If the leakage current is higher, but still relatively low, the feedback voltage is close to the open coil voltage and becomes difficult to distinguish from an open coil.
In certain solenoid control systems, such as, for example, Anti-Lock Brake Systems (ABS) and Electronic Stability Control (ESC) systems used in vehicle electronically controlled brake systems, the solenoids are utilized to operate valves that control the flow of brake fluid during a brake system operation. An unintended leakage current through a solenoid coil could result in a partial or complete opening or a closure of a valve, which could result in an undesired flow or an undesired blockage of brake fluid. If this condition does occur, it is necessary to switch off the high side FET 12 in order to stop the leakage current flow and prevent undesirable effects on brake system performance. Since the high side FET is common to all solenoid coils, switching it off also disables all of the solenoid coils, and therefore all electronically controlled brake system functions which require solenoid operation. In the case of an open solenoid coil, there is no undesired current flowing, so it is not necessary to switch off the high side FET. The other solenoid coils which are functioning properly are not disabled and therefore the only electronically controlled brake system functions which need to be disabled are those that require proper operation of the affected solenoid coil. Accordingly, it would be desirable to provide an apparatus and method to detect such small currents to distinguish a leakage current from an open solenoid coil.